Bridges which use an arch as its primary load bearing structure are for this category. Arch bridges can span up to roughly about 1700 feet. There are several bridges which use multiple forms of load bearing structures such as bridges using both an arch and truss, but if the bridge contains a prominent arch for support, then waymark it here. You can submit any arch bridge in this category whether it is for pedestrians, bikes, boats, cars or more. If you have waymarked a true stone bridge that has an arch, please also post it in the Stone Bridge Category.
Arch bridges use a semicircular structure to push the weight outward along the arch to the abutments. There are very few different types of arch bridges (as far as basic structural design); however, there are many different styles of arch bridges such as Roman, Baroque, and Renaissance, but they all structurally function the same. More information on arch bridges can be seen at this arch bridge link.
Here is how the load on an arch bridge is directed:
Below are some images of arch bridges:
| Solid ribbed arch (two hinged arch) |
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Spandrel braced (cantilever) arch |
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| Closed spandrel deck arch |
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Open spandrel deck arch |
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| Trussed deck arch |
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Trussed through arch |
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If you are familiar with what an arch bridge is, then proceed to the waymarking requirements and post your waymark. Otherwise, how do you know which bridge is which? Click on this link to see good diagrams explaining the many different types of bridges, and you can also read basics about them below. Another grood link to view for the different kinds of bridges is here.
Different types of bridges:
Here's a basic lesson to help you identify the different forms of bridges:
- Arch
- Beam and Girder
- Truss
- Cantilever
- Suspension
- Cable stayed
- Moveable
Arch:
Arch bridges are described above and are to be posted in this category. The other forms of bridges listed below are not to be posted in this category unless they include an arch that is a primary load bearing form.
Beam and Girder
These are probably the most widely used form of bridge construction due to its economy and simplicity over short distances. Beam or girder bridges are just that, made with long beams or girders that span horizontally across from one abutment to the other. Nothing more special about them. In modern bridges, "girder" is the term for the main load bearing beams, and "beam" for the smaller supports mounted across the girders. Girders and beams can look very similar to each other and are used interchangeably in this category description to keep it simple. A simple example of beam bridge: put two books on a table with a gap between them and lay two toothpicks across the gap. Then put a business card across the top of the tooth picks and you have just made your very own beam bridge. It may only hold a team of ants crossing it, but you made it anyhow. As you have probably guessed, these bridges can only span short distances between its abutments. You can read more about beam bridges at this link, and you can read about girder bridges at this link.
Here are examples of beam bridges:
Truss
These bridges mainly use a series of connected triangle shapes to form the structure. This structure then distributes the weight straight down on the abutments. Trusses distribute weight much differently than a cantilever bridge by having both of its ends bearing the brunt of the load. You can file truss bridges in the Truss Bridge Category. You can read more about truss bridges at this link.
These are some of the different styles of truss designs:
This is how the load of a truss bridge is distributed on the abutments:
Here are some examples of truss bridges:
Cantilever
This kind of bridge can look similar to the truss bridge construction at first glance, but it functions much differently. The cantilever actually balances on its one main vertical support post. In other words, it functions much like a lever or a teeter-totter. Think of it this way: balance a teeter-totter horizontally and have a person on each end standing on the ground supporting it; you now have a cantilever bridge. Many of these bridges use more than one cantilever to span greater distances. It takes a bit of looking to determine it from truss bridges, but after you have seen a few, you can then distinguish most of them. There are also cantilever bridges that do not use trusses, but use other materials such as concrete. You can read more about cantilever brides at this link.
Here are what some cantilever bridges look like:
Below is a concrete cantilever bridge under construction. This bridge is made with a concrete box girder form; however, cantilever is the major classification of the bridge. Notice how they started construction at the center pillar and moved outward on both sides evenly to keep the bridge balanced:
Suspension
The deck of this bridge is suspended from smaller cables or rods that tie into main horizontal cables (in some cases large metal links are used instead of cables). The main cables must be secured past the ends of the bridge to prevent the main posts from bending under the compression load of the deck. Suspension bridges are the best for spanning the longest gaps. You can file suspension bridges in the Suspension Bridge category. You can read more about suspension bridges at this link.
Here is a suspension bridge diagram:
This image shows a suspension bridge in the foreground and a cantilever bridge in the background. This suspension bridge also has a truss framework under the deck, but the main load bearing structure are the suspended cables:
Cable stayed
The newest boy on the block (if you consider 1952 new), is sometimes classified under suspension, but some people consider it different. We will let the engineers work that issue, but in the mean time we will go over what it is. This kind of bridge design has a series of cables that fan out from the center post to varying parts of the bridge deck. Each one of these cables independently supports its own portion of the deck and transfers that weight back to the main post. This is unlike a standard suspension bridge which the main horizontal cables carry the weight, and then has a subset of vertical cables that vertically support each respective portion of the deck. Even though cable stayed bridges usually use girders for under the deck, the cable stayed form is the primary load bearing structure, and therefore classified as such. You can read more about cable stayed bridges at this link.
Examples of cable stayed:
Moveable
If it is a bridge that moves to get out of the way of something…well, then it is a moveable bridge. Most of these are low bridges over waterways to allow shipping traffic to pass through. You can file moveable bridges in the Moving Bridges Category.
Then there are bridges that use combinations of these structures.
I am sure there are engineers that will dispute some of my descriptions, but I tried to keep it simple.